BuildPlanStrategies

I've found that method RemoveAllExtension() does not clear a BuildPlanStrategies (StrategyChain).
Accordingly at use of the extensions these strategies turn out doubled and IL too.
In my opinion it is a bug.

I think it will be better to add methods in StagedStrategyChain<TStageEnum> for their association and removal (Merge and UnMerge).
And UnityContainerExtension should have the StagedStrategyChain<TStageEnum> which it can easy remove if the given Extension leaves from the container.
Then there will be no problems with duplicates of strategies. After all strategy can be added and from other Extension.